The present invention relates generally to silicon carbide (SiC) whisker-reinforced ceramic composites, and more particularly to such composites and a method of preparing them wherein the ceramic material forming the matrix is mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) or alumina (Al.sub.2 O.sub.3) with particulate zirconia (ZrO.sub.2) added to the matrix for forming a silicon carbide (SiC) whisker-reinforced ceramic composite which provides advantages over SiC whisker-reinforced mullite and alumnia ceramic composites.
Ceramic composites reinforced with SiC whiskers have been receiving increased interest in applications requiring structured properties such as high resistance to chemicals, wear and errosion, and good structural integrity at high temperatures such as present in heat engines and the like. Reinforced ceramics are being seriously considered as candidates as structural materials for use in the fabrication of various heat engine components such as turbo-charger rotors, cylinders, bearings, valves, as well as other components of heat engines which will enable the heat engines to run more efficiently at higher temperatures than previously possible with similar components formed of metal. The reinforcing of various refractory oxides such as mullite or alumina with SiC whiskers provided a substantial increase in the fracture toughness of the ceramic so as to effectively inhibit deleterious crack formation and crack growth due to material fatigue. Such SiC whisker-reinforced ceramics are described and claimed in assignees' U.S. Pat. No. 4,543,345 which issued Sept. 24, 1985 and is entitled "Silicon Carbide Whisker Reinforced Composites and Method for Making Same". In assignees' patent, the ceramic composites exhibit increased toughness and resistance to cracking and are prepared by forming a homogenous mixture of particulate ceramic material such as alumina, mullite or boron carbide (B.sub.4 C) and about 5 to 60 vol. % of SiC whiskers. These whiskers have a monocrystalline structure and are of a size of about 0.6 micrometer in diameter and of a length of a range of about 10 to 80 micrometers. The SiC whisker-ceramic particulate mixture was hot pressed at a pressure in the range of about 28 to 70 MPa and at a temperature at a range of about 1600.degree. to 1900.degree. C. for a duration of 0.75 to 2.5 hours for providing a SiC whisker-reinforced composite with a density greater than about 99 percent of the theoretical density of the composite matrix material. Assignees' aforementioned patent is incorporated herein by reference.
The SiC whisker-reinforced ceramic composites provided by the practice of the invention described in assignees' patent briefly described above provided articles with a significant increase in fracture toughness over those provided by conventional ceramics. It has also been found that SiC whisker-reinforced ceramic composites utilizing mullite as the matrix material provide several advantages over the ceramic composites using alumina as the matrix material. For example, mullite-based composites with the SiC whiskers exhibit lower thermal conductivity (20 percent of that of similar based composites with Al.sub.2 O.sub.3); lower coefficient of thermal expansion (about 55 percent of that of similar Al.sub.2 O.sub.3 based composites); better thermal shock resistance; excellent creep resistance; and lower density and thus potentially lighter weight.
Recent modeling of the toughening behavior of ceramics with unstabilized or partially stabilized zirconia additions reveal that such additions to SiC whisker-reinforced ceramics should substantially improve the mechanical properties of both the mullite and the alumina-based composites. In the utilization of zirconia where a sufficient quantity of a stabilizing agent such as yttria has been added to fully stabilize the ZrO2 to a cubic phase, it has been found that the SiC whiskers undergo deleterious degradation in the composite apparently due to a reaction with the yttria stabilizer.